In certain people at certain times, electrical signals within heart tissue may not function properly and can create cardiac arrhythmias. Ablation of cardiac conduction pathways in the region of tissue where the signals are malfunctioning may reduce or eliminate such faulty signals. Ablation involves creating lesions on tissue during surgery. To provide effective therapy, surgically created lesions may block the transmission of cardiac contractions.
Ablation may be accomplished in several ways. Sometimes ablation is necessary only at discrete positions along the tissue, is the case, for example, when ablating accessory pathways, such as in Wolff-Parkinson-White syndrome or AV nodal reentrant tachycardias. At other times, however, ablation is desired along a line (either straight or curved), called linear ablation. (In contrast to linear ablation, ablations at discrete positions along the tissue are called non-linear or focal ablations.) One way is to position a tip portion of the ablation device so that an ablation electrode is located at one end of the target site, which may be a lesion line. Then energy is applied to the electrode to ablate the tissue adjacent to the electrode. The tip portion of the electrode is then dragged or slid along the length of the desired lesion line while delivering energy. A second way of accomplishing linear ablation is to use an ablation device having a series of spaced-apart band or coil electrodes that, after the electrode portion of the ablation device has been properly positioned, are energized simultaneously or one at a time to create the desired lesion. If the electrodes are close enough together the lesions run together sufficiently to create a continuous linear lesion.
Typical areas of the heart that are treated using surgically created continuous linear lesions are located in the atria. This may be the case for atrial fibrillation, which is a common form of arrhythmia. The aim of linear ablation in the treatment of atrial fibrillation may be to reduce the total mass of electrically connected atrial tissue below a threshold believed to be needed for sustaining multiple reentry wavelets. Linear transmural lesions may be created between electrically non-conductive anatomic landmarks to reduce the contiguous atrial mass. Transmurality is achieved when the full thickness of the target tissue is ablated.
In a procedure to treat atrial fibrillation, the pulmonary vein ostia may be isolated on a beating heart from the left atrium using a linear bipolar ablation clamp device. The linear clamp device often delivers radio frequency energy between the linear electrodes, which may heat and kill the portion of myocardium that is clamped between the two electrodes. This may form a continuous line of inactive myocardium encircling that portion of the heart. With this method, all tissue distal to, or away from, the heart may be isolated from the normal heart contractile function.
Before the procedure is complete, the area of the heart may be tested to confirm a conduction block or see if the ablation is effective and eliminates the undesired electrical signals. Present methods to confirm conduction block include the use of electrophysiology catheters to evaluate pulmonary vein isolation lesions and monopolar and bipolar focal probes using pacing or electrogram techniques, and are described below.
Surgeons may use multi-polar electrophysiology catheters to evaluate pulmonary vein isolation by manually placing the catheters on the lesion or on one side or the other of the lesion. The catheters may be used to provide information that will display bipolar electrograms from electrode pairs on the catheters for analysis. This can be done using an electrophysiology recording system or a portable pacemaker programmer/analyzer. In these cases, the surgeon removes the ablation device from the patient and then replaces it with the catheters or the probes. This can be cumbersome in that medical professionals may prefer the simplicity of a single surgical instrument with which to perform both ablation and lesion evaluations whenever possible.
Additionally, procedures on a beating heart may be preferred over procedures on an arrested or stopped heart. Holding a catheter designed for endocardial use on the epicardial surface in a stable location on a beating heart may be challenging. The catheters may be held against the epicardial surface with forceps where the surgeon provides a force to keep the catheter steady against the beating heart. In addition to having to maintain a constant position placement of the catheter, the catheter can be misplaced. Only the active myocardium below each pair of electrodes may be detected. If the electrodes are not positioned correctly, the results can falsely indicate that isolation has been achieved. In the same way, pacing can be used from an electrode pair to evaluate if there is an effective conduction block.
In addition to the catheters, probe-type devices can be used to evaluate cardiac conduction block using pacing or local electrogram sensing techniques. For example, a monopolar pacing device can be used to evaluate conduction block by placing a ground electrode needle into the intercostal muscle of the chest, which is also connected to the positive pole of the pacing pulse generator. A detector probe may be connected to the negative pole of the pacing pulse generator. The detector probe can be used to apply pacing stimuli to discrete locations within a region of tissue isolated from the ablation. The region can be considered isolated if pacing from the region does not produce ventricular capture. Failure to capture the tissue under the probe tip, however, could also be due to a lack of myocardium in that specific area. In addition, myocardium could be present and remain undetected in other adjacent areas within the areas intended for isolation. Thus, even though the probe may appear to be positioned properly, if it is not positioned directly adjacent to a portion of the lesion that failed to create a complete block, the probe may fail to register the existence of the incomplete lesion. This is because while the portion of the lesion it is adjacent to may be complete, other portions of the lesion may not be complete.
Other probes may be designed for monopolar and bipolar focal ablation and also bipolar pacing and electrogram sensing. In a similar way to the above-described bipolar probe, the probes may not cover much area on the tissue as they detect or stimulate active non-isolated myocardium. Probes may be placed on the myocardium that connects to the atrium to improve the likelihood that tissue adjacent to the probe is connected to the atrium. Thus, the probe may be moved over many portions of the isolated tissue to determine that no isolated regions exist. Further, the probes might not be the preferred ablation device of the medical professional, and so the selected ablation device may be removed and replaced with a new sensing tool, similar to the cumbersome procedures described above with the electrophysiology catheters.